A reliable superconducting link, resistant to thermal cycling, is necessary in the construction of multicomponent superconducting devices, such as multichannel magnetometers, where simultaneous faultless operation of several superconducting devices is required. Such devices are used, for example, in the detection of feeble biomagnetic signals from the human brain.
Superconducting contacts between electronic components fabricated by thin film technology on substrates, such as silicon wafers, quartz, sapphire or glass, are technically problematical for the following reasons. 1) The electrical contact must be superconducting which requires a clean metal-to-metal contact between the bonding wire and the thin film. 2) The differences in thermal contraction of the various construction materials (silicon, lead, tin, niobium, printed circuit board made of fiber glass) lead to considerable thermal stresses at the joints because, in order to achieve superconductivity, the devices must be cooled far below their fabrication temperature. 3) Fixing the broken contacts at the operation temperature is impossible. Therefore, especially in devices comprising a large number of superconducting components, the joints must be extremely reliable and resistant to thermal cycling so that, when warming up the device to fix a link, one does not break any of the other parallel joints.
The prior art of superconducting bonding technology utilizes lead, tin-lead or niobium wires because these materials are sufficiently soft for mechanical bonding and their superconducting transition temperatures are well above the liquid helium temperature (e.g. S. Kiryu et al. in Advances in Biomagnetism, eds. S. J. Williamson, M. Hoke, G. Stroink, M. Kotani, Plenum Press, New York 1990). Lead, tin and their alloys melt at low temperatures (200.degree.-400.degree. C.) so that the softening of these materials slightly below the melting point can be utilized in the bonding. Ultrasound rather than heating is used when bonding with niobium wires because the melting point of Nb is very high (2470.degree. C.). Compared to lead, niobium wire is also stiff so that to avoid too high stresses at the joints, either long or very thin wires (&lt;10.mu.m) must be used. Thin wires are difficult to handle and long wires come off easily during further handling of the bonded device. Long wires also require extra space and, especially in the magnetometer applications, form parasitic superconducting pickup loops. Tin and lead are too soft to be bonded with ultrasound. Wires made of these materials are usually pressed against the bonding pad of the thin film device by aid of a small, hot soldering tip. However, because typical substrate materials are good conductors of heat, these wires melt easily and get stuck on the soldering tip before the intended proper softening at the wire-thin film interface. The resulting joint may be mechanically weak and come off in cooling. Or, it may not be superconducting throughout. Even if the bonding is successful, the stress at the joint, caused by the relatively thick wire (40-100.mu.m), may break the bond during cooling.